The invention relates in general to key input circuits for portable electronic devices and the like, and in particular to the interface between a key input apparatus and a key input decoder.
Many devices exist which use a key input apparatus, such as a keyboard or keypad, to provide input data. A difficulty in implementing a full-function key input apparatus in a small space, such as in a portable, hand-held electronic device, is minimizing the size of the keys or key space used to contact the key matrix network commonly used to provide signals to the device from the key input apparatus. Conventional portable electronic units typically utilize conductors in a mesh arrangement on a printed circuit board or membrane-type material to provide the signal interface from the key input apparatus to a key decoder in the unit. The matrix pattern of the mesh arrangement is usually very complicated and requires a large surface area, particularly as the number of keys in the keyboard increases.
Another disadvantage associated with the use of a matrix-type key interface network is that the number of input/output ports required between the matrix and the key decoder is large, usually at least one-half the number of keys on the key input apparatus. Thus, a 16-key input unit for such an electronic device would require at least eight input/output ports on the decoding unit. As the number of required keys increases, the number of necessary input/output ports increases proportionately. Because the decoding unit is typically a microcontroller, the number of required ports greatly impacts not only the size but the cost of the device.
There is therefore a great need for an interface design for a key input apparatus that takes up a minimum amount of space in an electronic device, has a less complicated design than conventional matrix networks, and which minimizes the impact of input/output port allocation of a key decoder within the device.
It is therefore an object of the invention to provide an interface for a key input apparatus that requires less physical space than does a conventional key interface.
It is a further object of the invention to provide an interface for a key input apparatus that has a simple design.
It is an additional object of the present invention to provide an interface for a key input apparatus that can interface with a key decoder using a minimum number of input/output ports on the decoder.
These and other objects of the invention are achieved by an interface for a key input apparatus to an electronic device, which includes a sense resistance having an input point for connection to a voltage source and a resistive element having a number of contact points, and a ground point for connection to a ground potential. The interface also includes a number of key switches connected to the sense resistance. Each of the number of key switches corresponds to one of the number of contact points, and the key switches are each disposed such that when one of the key switches is asserted, an electrical connection is made between the corresponding contact point and the input point, forming an electrical path from the input point to the ground point through the sense resistance, the asserted key switch, and at least a portion of the resistive element. The interface also includes a sampling point on the electrical path for providing a sampling voltage to a decoder of the electronic device when one of the number of key switches is asserted.
According to one aspect of the invention, the sampling point is a point at which the number of key switches is connected to the sense resistance. Alternatively, the sampling point may be the ground point. The resistive element may have a resistance that is uniform over a length of the resistive element, or the resistive element may have a resistance that is scaled over a length of the resistive element. Likewise the contact points may be uniformly spaced along a length of the resistive element, or the contact points may be spaced along a length of the resistive element according to a non-uniform scale.
According to a further aspect of the invention, the resistive element has a first portion over which all the number of contact points are spaced, and a second portion having a resistance that is at least as great as a resistance of the first portion. The ground point may be a first ground point, in which case the resistive element has a second ground point for connection to a ground potential. The interface may further include a bias resistor connected to the resistive element and for connection to a ground potential. The bias resistor may include a bias point for application of a bias potential.
According to another embodiment of the invention, the interface for a key input apparatus to an electronic device includes a sense resistance having an input point for connection to a voltage source, a resistive element having a number of contact points, a first bias resistor connected to the resistive element and for connection to a ground potential, and a second bias resistor connected to the resistive element and for connection to a ground potential. The interface also includes a number of key switches connected to the sense resistance. Each of the key switches corresponds to one of the number of contact points, and the key switches are disposed such that when one of the key switches is asserted, electrical connection is made between the corresponding contact point and the input point, forming an electrical path from the input point to the ground point through the sense resistance, the asserted key switch, and at least a portion of the resistive element. The interface further includes a sampling point on the electrical path for providing a sampling voltage to a decoder of the electronic device when one of the number of key switches is asserted.